/Engine/Source/Runtime/SignalProcessing/Public/DSP/Dsp.h
When called, seeks the read or write cursor to only retain either the NumSamples latest data (if bRetainOldestSamples is false) or the NumSamples oldest data (if bRetainOldestSamples is true) in the buffer. Cannot be used to increase the capacity of this buffer.
void SetNum ( uint32 NumSamples, bool bRetainOldestSamples )